Field of the Invention
The present invention relates generally to improved brightness and contrast in image projection systems. Specifically, the invention relates to brightness and contrast improvements in liquid crystal light valve based video projection systems.
The demand for large screen video is growing rapidly as VCR, Laserdisc and computer driven programming are used increasingly by organizations of all sizes for all types of applications. Large screens, as opposed to monitors, are needed when the application calls for more than a few persons to look at the same video screen. All large screens (over 35 inches) are either rear or forward projector units. The vast majority of all projectors sold are based on a design that uses 3 high-power CRT's (one for each primary color) to both form the image and to provide the actual projection light. This is accomplished by merely focusing each of the 3 CRT's onto the viewing screen with 3 separate projection lenses.
The basic problem with CRT based video projectors is brightness. A CRT's brightness is proportional to the size of the CRT's screen and to the power supplied to the CRT. Thus brightness can be increased by either increasing the size of the CRT or by increasing the power supplied to it. But as more power is provided, heat becomes a serious problem and image resolution suffers. In addition, as screen sizes increase, the optics become prohibitively expensive. Currently 9 inch diagonal CRT's can deliver approximately 300 lumens to the screen. When projectors having this level of brightness are used on a 60 inch screen, the ambient light in the screening room must be very dim for the picture to be seen. And if a larger screen is used, the room must be proportionally darker. But, in a large number of applications such as training seminars, it is an advantage to have a bright screening room.
The most promising approach to solving the brightness problem comes from a new projector technology, light valves. This technology holds great promise, but is not now commercially practical (except for a few very expensive units sold in small quantities). The present invention is intended to solve some of the problems that have prevented the light valve projector from becoming a widely available commercial success.
In general, light valve based video projectors work as follows. White light from a high-intensity source, such as a Xenon lamp, is separated into component primary color beams and polarized. Each polarized primary color light beam is relayed through a series of lenses and mirrors to a valve which then modulates the polarization phase of the light as it is reflected from the light valve. The high-intensity light pattern in each color channel is analyzed by polarizer-analyzer optics and then projected onto a viewing screen. Low power CRT's are used to address the liquid crystal light valves.
Projector light output determines as a practical matter both screen size and how bright the screening room may be since a good picture is partly a function of the difference between the brightness of the on screen image and the level of light in the room. But image quality is also strongly affected by the difference between the brightest and darkest parts of an on screen image regardless of the level of light in the room. This difference is called contrast ratio. The higher the contrast ratio, generally speaking, the more pleasing the projected image.
One of the salient problems standing in the way of the commercialization of light valve projectors is as follows. It is particularly difficult, yet necessary, to achieve a high contrast ratio and high brightness and at the same time have an efficient optical system. The only suppliers of commercial light valve based projectors have approached this problem by sacrificing efficiency. These suppliers make projectors that have a huge projection light source. But the optics of the system are such that only a small portion of the available light is used. This design yields a projector with high contrast. It also yields a projector with brightness that in spite of the inefficiency is much greater than that of CRT based projectors. The problems with this approach are at least twofold. First, the projectors are large, heavy and expensive. Second, they require at least 220 volt power which inherently means a custom, fixed installation. However, most users need the flexibility to wheel the projector from room to room and plug into widely available 110 volt office power supplies.
These problems with light valve projectors have been obvious for years and many have attempted to solve or at least improve on the situation.
This problem was addressed in U.S. Pat. Nos. 4,191,456 by Hong, and 4,464,018 by Gagnon. Their approach was the addition of a reflective pre-polarizing prism of the MacNeille type to the polarization optics of the high-intensity projection light. Light of one polarization state is transmitted into the optics train while light of the second polarization state is discarded from the system. This approach improves the contrast ratio because the introduction of the prepolarizing prisms improves the polarizing efficiency of the system. The problem with this approach is that the system throughput efficiency is limited by the initial rejection of 50% of the input light. Thus brightness and efficiency were sacrificed.
To enhance contrast and improve throughput efficiency in a two-color projection system, Gagnon, U.S. Pat. No. 4,500,172 describes the use of a prepolarizing prism to transmit light of a first polarization state to a selective color filter which reflects light of the first polarization state and first color to a beam combiner and hence to a second polarizing prism to be transmitted to a first liquid-crystal light valve. Light of the second polarization state is reflected from the prepolarizing prism to a selective color filter where light of a second color is reflected to the beam combiner and hence transmitted to the polarizing prism to be reflected to a second liquid crystal light valve. In this way, the light in both polarization states is preserved. However, this design requires that the polarizing prisms maintain polarization efficiency over the passbands of both the first and second colors. To accomplish this requires additional processing time and expense to deposit the additional polarizing layers. In addition, this design is limited to two colors.
To extend the two color design of U.S. Pat. No. 4,500,172 to full color, Gagnon, U.S. Pat. No. 4,425,028, described a fluid-coupled optical tank with color selective pre-polarization. This is a complicated optical design. It calls for plate prepolarizing beam splitters which require as many as 15 thin film layer pairs. In addition, the optical system is immersed in a high refractive index fluid. This system selectively rejects light of a first color passband and first polarization state while transmitting light of first color and first polarization state as well as light of second and third colors with first and second polarization states. The advantage of this design is that it allows high contrast and is compact in size and configuration. A disadvantage is that as in the previous design, polarization splitting of the "S" and "P" polarization states by the dichroic filters and polarizers reduces the color passband ranges and consequently causes low throughput and loss of brightness.
In spite of the foregoing efforts, little progress has been made in solving the problem of providing high contrast and brightness in a projector that can be run from a 110 volt power source.